To send a payment in iMessage, you need to have an iPhone, the latest version of the Venmo app, and the latest version of iOS (iOS 13 or later) on your device.
If you’ve downloaded iOS 13 or a later version and you’re signed into your Venmo account, you can send a payment to other Venmo users in iMessage.
If the person you’re trying to pay doesn’t have a Venmo account, you can start a payment by sending it to that person in iMessage, but they will need to sign up for Venmo to accept the payment. The recipient will also need a web-enabled smartphone that can receive text messages (but they don’t necessarily need to be on an iPhone).
Payments sent in iMessage must be accepted by the recipient and are not automatically added to the recipient’s Venmo account. Please note that payments sent in iMessage expire if they are not accepted after 3 days. At that point, the money is returned to the sender.
Please ensure that you’ve enabled the iMessage extension for the Venmo app. If you haven’t, you can do so by tapping 
--> tap 
--> tap 
in the bottom lefthand corner of the keyboard --> tap 
--> toggle Venmo to be green.